all sky in the far infrared: first results from the akari all sky survey agnieszka pollo ipj...

All sky in the far infrared: first results from the AKARI All Sky Survey

Agnieszka PolloIPJ

Warszawa, 12.05.2010

Electromagnetic spectrum

Infrared range: longer than optical and shorter than microvawe waves.

Infrared

Astronomers roughly divide infrared into three ranges:

1. near- (NIR: 1 – 5 micrometers),

2. mid- (Mid-IR: – 5 -- 30 micrometers)

3. far- (FIR: 30 – >200 micrometers).

Infrared = heatAll objects in the Universe with ANY temperature radiate in the infrared

Humans in the Infrared

Human body of a normal temperature has radiates with a maximum in the infrared around 10-12 microns.

Infrared astronomical observations

Astronomy: observations in the infrared

Atmosphere– absorbs infrared– emits in the infrared itself

Atmospheric emission is the strongest at ~10 μm

There are a few IR “windows” in the atmosphere where there is no emission and no strong absorption, mainly above ~ 4 μm (NIR).

Infrared windows in the atmosphere

Band Sky Transparency J high low at night H high very low K high very low

L low

M low high

N very high

17 - 40 microns very low very high

WavelengthRange

Sky Brightness

1.1 - 1.4 microns 1.5 - 1.8 microns 2.0 - 2.4 microns

3.0 - 4.0 microns

3.0 - 3.5 microns: fair3.5 - 4.0 microns:

high 4.6 - 5.0 microns

7.5 - 14.5 microns

8 - 9 microns and 10 -12 microns: fair

others: low 17 - 25 microns: Q28 - 40 microns: Z

Astronomical observations in the infrared

Telescopes in high dry mountains (Atacama)

airplanes

balloons

satellites

What can we observe in IR?

Everything hidden behind dust

Everything cold:– dust – cold stars– planets

Everything (?) far: strongly redshifted galaxies

Spitzer: star forming regions in Cygnus


Everything hidden behind dus



Spitzer, “hot Jupiter” HD 189733b

650oC 930oC


Hubble Deep Field in NIR

Spitzer:cosmic IR background from very first galaxies?

Everything hidden behind dus




Astronomical objects in IR look different than in other wavelengths

Different parts of the spectrum show different things: Far IR: dust, UV: young hot stars optical: most of stars which are not obscured by

dust near-IR: stars hidden behind the dust (here the

dust becomes relatively transparent)


This makes IR a very important range for galaxy observations

– it allows to see the parts of galaxies which are completely hidden by dust (and sometimes whole galaxies faint or invisible in optical range) – important for a total census of stellar light (and mass) in the Universe

– it gives a possibility to discover very distant galaxies

Copyright by: Kasia Małek

Orion in optical and IR

M31 (Andromeda)

optical

FUVFIR

IRAS

Satellite IR observatories

First IR satellite, launched by NASA in January 1983

First ever map of (almost - 98%) all sky in IR during a ten month period from January to November, 1983

All sky in IR - IRAS (80')

IRAS – All Sky in IR 60 cm helium-cooled

telescope 4 IR bands at effective

wavelengths: 12, 25, 60, 100 μm

The angular resolution varied between about 0.5' at 12 microns to about 2' at 100 microns

After a 10 month long mission, IRAS exhausted its cryogen and ceased operations on November 21, 1983

IRAS – All Sky in IR ~ 350 000 IR point sources

in the sky which increased the number

of cataloged astronomical sources of 70%

most of them belong to Milky Way: cool stars, nebulae, cirruses...

plus a few tens of dusty galaxies

some sources still remain unidentified

AKARI

• 68.5 cm diameter telescope• two main instruments:

– the Infrared Camera (IRC) – for mid-IR– the Far-Infrared Surveyor (FIS) – for FIR

• launched in February 2006• 16 month cryogenic mission lifetime

between May 2006 and August 2007 (needed for FIR observations; liquid helium ran out on 26 August 2007 )

• now – the “warm” phase • deeper; much better resolution than IRAS

AKARI

6 IR bands from 9 to 180 μm (broader range than IRAS and reaching longer wavelengths)

Planned: All Sky Survey + two deep surveys (NEP and ADF-S) + a series of dedicated pointed observations

Improvement of resolution comparing to IRAS

In MIR

In FIR

Akari All Sky Surveys: point source catalogs at FIR and MIR

• public release 31 March 2010• (not yet crossed-matched)• first (simple) science results published in a

dedicated A&A special issue

Akari All Sky Surveys: point source catalogs at FIR and MIR

• in total, more than 1.3 mln sources (> 3 times more than IRAS) in 6 bands

• AKARI-IRC Point Source Catalogue v. 1:– 870 973 objects in two MIR bands (9 and 18

μm) – 10 times more sensitive (at 18 μm) than

IRAS– an accuracy of arcseconds (compared to

arcminutes with IRAS)• AKARI-FIS Bright Source Catalogue v. 1:

– 427 071 sources in 4 FIR bands (65, 90, 140, and 160 μm)

– (IRAS longest band was 100 μm)

Infrared sources at 9 μm: blue, at 18 μm: green, at 90 μm: red.

Galactic center Galactic plane

AKARI All-Sky survey at 9 μm

• Emission from the photospheres of stars dominates the 9 μm catalogue: the galactic disc and nuclear bulge are clearly visible at this wavelength

NEP (North Ecliptic Pole)

ADF-S (South Ecliptic Pole, AKARI Deep Field South)

AKARI All-Sky survey at 9 μm

Infrared sources at 9 μm: blue, at 18 μm: green, at 90 μm: red.

Galactic center

Galactic plane

•dust and star formation in the disc of our Galaxy become more prominent at 90 micrometres;

•Away from the Galactic Plane, many extragalactic objects are detected

FIR: AKARI ASS (AKARI All-Sky Survey: Bright Source Catalog)

v. β-1: 94% of the sky in 16 months

>43 000 sources with fluxes measured in all four FIS bands (160, 140, 90, 65 μm), i.e. “colors”

What are these sources?

• Statistical analysis of all sky surveys provides a powerful tool to understand the properties of all classes of objects in the Universe.

• But first, we need to know: what they are?• From our point of view, the crucial point was:

which of these sources are the galaxies, how they can be distinguished from sources which belong to Milky Way?

• If, e.g., we want to make a (costly) measurement of galaxy distances by spectrophotometry, we do not want our sample to be “poluted” by too many stars (and vice versa, stellar researches do not want to be bothered by galaxies).

What are these sources?

• In case of FIR studies there is no credible way to find good galaxy candidates (yet)

• At first, we have at our disposal only FIR fluxes (i.e. FIR colors)

Preceding Study from IRAS

With IRAS four bands (12, 25, 60, 100 μm), a very detailed classification was possible. However, in the case of AKARI FIS ASS, we must rely only on four FIR bands (at longer wavelengths), and this cannot be a trivial application of IRAS methodology, since the physical processes behind emission in these bands are different.

(Walker et al. 1989)

Classical method: color-color diagrams.

The color-color diagrams

• The basic idea: different classes of astronomical (and not not only) objects have different colors

• Color is defined as a difference between fluxes at different wavelengths (also far from optical)


• Such differences were first observed in the optical range: it is well known that, e.g. young stars are bluer than old ones, and spiral galaxies are bluer than ellipticals.


• This is (broadly speaking) related to the fact, that different object have different spectra, and their shape may in a complex way vary depending on their properties

Here: templates from Buzzoni at al. 2005

1. The sample was matched with SIMBAD and NED (astronomical database for stars, nebulae, and galaxies).

Star-Galaxy Separation by FIS Color-Color Diagrams

Data

1. Since we were looking mainly for galaxies, we selected sources in a low-cirrus region (I100 < 10 MJy sr-1) on the sky to avoid contamination in FIR flux (5176 objects), which in practice meant mainly avoiding Galactic plane.

Objects in the All Sky Survey

• In this way, we found– 4272 galaxies– 382 other

extragalactic objects– 399 Galactic objects– among them, 349

Milky Way stars– for 101 sources it

remains unclear whether they are Galactic or not

– only 22 sources were left unidentified

Color-color diagram (an example)

We found that we can define a separation line on practically all the FIS color-color plots to select >97% of galaxies and reject > 80\% of stars. (Pollo, Rybka & Takeuchi, 2010, A&A).

galaxies

stars

Other and unidentified objects

Only sources with the best photometry:

stars (green)

galaxies (red) other

(violet)

Star-galaxy separation in the color-color plots

• Color-color diagrams allow for a very good star-galaxy separation

• Stars form two branches: – a bigger, “bluer” branch is dominated by optically

bright stars, mostly evolved stars and pulsating variables (often Mira-type)

– a smaller branch overlapping galaxies contains a few bright stars with known IR excess (due to, e.g. dusty disks) – most notable among them is Vega, and a certain number of stars optically very faint, usually known before only thanks to their IR identifications (IRAS, 2MASS) – these stars would be in any case very difficult to be distinguished from galaxies only from FIR colors

Star-galaxy separation in the color-color plots

• Our method allows for a good separation of galaxies from stars – the contamination of a “blue branch” of stars by galaxies is very low

• This applies to FIR-bright objects from outside of the Galactic plane

• Most of the observed galaxies (with known z) are nearby galaxies (z<0.1) – however, we expect that more distant galaxies should be even redder – the method should remain valid

How does it apply to the Galactic plane

• In the remaining part (i.e. disk of the Galaxy):

– much more unidentified sources (40% vs 0.5% in the analyzed part) – this is probably related to much better resolution of AKARI with respect to previous experiments

– much less galaxies (15% vs 80%)– similar percentage (!) of stars and nebulae –

again, the reason is most probably the limited resolution of previous observations

– much more sources objects of unknown nature (observed before but not identified) – 30% vs 3%

– classification of objects from the Galactic plane will require more and much more careful analysis

all sky in the far infrared: first results from the akari all sky survey agnieszka pollo ipj...

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